1. Field of the Invention
The present invention relates to an ion-conducting, sulfonated and crosslinked copolymer for use as a polymer electrolyte membrane for a fuel cell and a fuel cell comprising the same. More particularly, the polymer electrolyte membrane comprises an ion-conducting, sulfonated and crosslinked copolymer which may reduce swelling, may reduce methanol crossover, may provide excellent dimension stability, and may be capable of maintaining high ionic conductivity.
2. Description of the Related Art
Fuel cells are electrochemical devices which directly convert the chemical energy of oxygen and hydrogen from hydrocarbon substances such as methanol, ethanol, or natural gas into electrical energy. Since the energy conversion processes of fuel cells are very efficient and environmentally friendly, fuel cells have attracted attention.
Fuel cells may be classified into phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFCs), polymer electrolyte membrane fuel cells (PEMFCs), alkaline fuel cells (AFCs), and other types according to the type of electrolyte used. Although these fuel cells operate on the same basic principles, they differ from one another, for example, in terms of the type of fuel used, the operating temperature, the catalyst used, and the electrolyte used. PEMFCs are popular not only for small stationary electricity generating apparatuses, but also for transportation systems due to their low operating temperature, high power density, short start-up time, and quick response to changes in power requirements.
A key portion of a PEMFC is a membrane electrode assembly (MEA). The MEA generally consists of a polymer electrolyte membrane and two electrodes attached to both sides of the polymer electrolyte membrane, the electrodes functioning as a cathode and an anode, respectively.
The polymer electrolyte membrane functions as a separator layer which prevents the direct contact of an oxidizing agent with a reducing agent and functions to electrically isolate the two electrodes. Further, the polymer electrolyte membrane functions as a proton conductor. Thus, an excellent polymer electrolyte membrane must generally provide: (1) high proton conductivity, (2) high electrical insulation, (3) low permeation of reactants, (4) excellent thermal, chemical, and mechanical stability under fuel cell operating conditions, and (5) low costs.
To satisfy the above requirements, various polymer electrolyte membranes have been developed. Currently, highly fluorinated polysulfonic acid membranes, made from a material such as NAFION™ (Dupont), are often used due to their excellent durability and performance. However, a NAFION™ membrane should be sufficiently humidified to operate well, and should be used at 80° C. or less to prevent loss of water.
In DMFCs, an aqueous solution of methanol is provided as a fuel to an anode. A portion of an unreacted methanol solution penetrates the polymer electrolyte membrane and induces swelling of the polymer electrolyte membrane, while diffusing and being transported into a catalytic layer of a cathode. Such a swelling phenomenon is referred to as ‘methanol crossover’. When methanol crossover occurs, methanol is directly oxidized at the cathode where an electrochemical reduction of hydrogen ions and oxygen normally occurs. Thus, an electrical potential of the cathode drops, and as a result, the performance of the fuel cell may be seriously deteriorated. This problem similarly occurs in other fuel cells that also use an organic liquid fuel with polar qualities.